Anomalous multi-order Raman scattering in LaMnO₃: a signature of quantum lattice effects in a Jahn–Teller crystal

Abstract

The multi-order Raman scattering is studied up to fourth order for a detwinned LaMnO3 crystal. Based on a comprehensive data analysis of the polarization-dependent Raman spectra, we show that the anomalous features in the multi-order scattering could be the sidebands on the low-energy mode at about 25 cm(-1). We suggest that this low-energy mode stems from the tunneling transition between the potential energy minima arising near the Jahn-Teller Mn(3+) ion due to the lattice anharmonicity and that the multi-order scattering is activated by this low-energy electronic motion. The sidebands are dominated by the oxygen contribution to the phonon density-of-states, however, there is an admixture of an additional component, which may arise from coupling between the low-energy electronic motion and the vibrational modes.

Optical evidence of quantum rotor orbital excitations in orthorhombic manganites

“…Here we note that depopulation of the electron-vibrational levels with energies of about 16 and 32 meV can drive the orbital structure instabilities below about 200 and 380 K, which is in agreement with the observations by Murakami et al [12]. We further note that we had observed the excitation at about 80.6 meV (650 cm −1 ) in our recent Raman study of LaMnO 3 , and described it in the cited paper as a feature of unknown origin [15].…”

“…The key parameter here is α, which is used in the quantisation of the discrete energy levels. The observed narrow quantised rotational states were best described by the parameter α = 21 cm −1 , which is in good agreement with the value of the tunnelling splitting α estimated at about 25 cm −1 from our independent study of anomalous Raman scattering in LaMnO 3 [15]. On the other hand, we see dynamic-to-static orbital effects, stimulated by the superexchange interaction around the Néel temperature.…”

“…The parameter α = 21 cm −1 was used, which is in good agreement with the value of the tunnelling splitting α estimated at about 25 cm −1 from our recent study of anomalous Raman scattering in LaMnO 3 [15]. We calculated the following rotor excitation energies corresponding to the modulus of the angular momentum quantum numbers j given in brackets: 0.65 meV (1/2), 16.3 meV (5/2), 31.9 meV (7/2), 78. ,... .…”

“…These changes are accompanied by softening of the Raman-active phonon modes [13,14,15]. Similar behaviour arises in another classic example of orbital physics, KCuF 3 .…”

“…Then, the coupling of e g − e g excitation with the quantum rotor states could correspond to the coherent long-lasting exciton propagation, influenced by the anisotropic spin fluctuations around the Néel temperature. The existence of orbital excitations is still very controversial and currently is causing many debates in the scientific community due to the lack of conclusive experimental evidence [15,19,20,21]. Here we hope to offer new experimental data in support of this hypothesis.…”